add test to ast: test the Stringer-function

[Tiffilore]

The Stringer-fct for asts basically mirrors the input - with some brackets for clarification.

In some cases, however, it is not readable any more.

expl:

>> let remainder = fn(x, y) { if(y > 0){let p = (x/y) * y; x - p}}
let remainder = fn(x, y) if(y > 0) let p = ((x / y) * y);(x - p);

---

more general:

>> if(cond){a;b} 
ifcond ab
>> if(cond){a}; b 
ifcond ab

---

it's also not always valid Monkey code:

>> let remainder = fn(x, y) { if(y > 0){let p = (x/y) * y; x - p}}
let remainder = fn(x, y) if(y > 0) let p = ((x / y) * y);(x - p);
>> let remainder = fn(x, y) if(y > 0) let p = ((x / y) * y);(x - p);
            __,__
   .--.  .-"     "-.  .--.
  / .. \/  .-. .-.  \/ .. \
 | |  '|  /   Y   \  |'  | |
 | \   \  \ 0 | 0 /  /   / |
  \ '- ,\.-"""""""-./, -' /
   ''-' /_   ^ ^   _\ '-''
       |  \._   _./  |
       \   \ '~' /   /
        '._ '-=-' _.'
           '-----'
Woops! We ran into some monkey business here!

Test (in pseudocode):

let ast1 = result of parsing input
let mirror = ast1.String()
let ast2 = result of parsing mirror

Then we want: ast1 == ast2